5 Use of terminal adaptation functions

3GPP43.045Release 17Technical realization of facsimile group 3 transparentTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

According to the Connection types (figure 4/43.045) there are two classes of TAFs to be considered.

5.1 Standard TAFs for synchronous services

Are those described in 3GPP TS 27.003 for synchronous bearer capabilities in the transparent mode. The Rate Adaption functions shall comply with the 3GPP TS 44.021.

The interchange circuit signalling mapping is in accordance with the 3GPP TS 27.003.

5.2 Specific TAFs for facsimile service

Integral part of an end‑to‑end connection for this Teleservice is the Fax Adaptation function, located at both the PLMN ends and in charge of adapting the T.30 protocol procedure to the PLMN environment, as described in clause 4 of the present document.

The main features relevant to the adaptation function are detailed in the following. For a better clarification, the Finite State Machine approach will be adopted in the following description of FA functionality, considering it as a set of states defined by specific interactions with both the PLMN digital channel and the analogue channel.

5.2.1 Working principle

The working principle of the proposed solution is depicted in figure 6/43.045.

Figure 6/43.045: Fax Adaptor working principle

The Fax Adaptor is split in its main functional blocks:

– a BCS (300 bit/s) modem capability (half duplex);

– a MSG (14400/12000/9600/7200/4800/2400 bit/s) modem capability (half duplex);

– Mux/Demux capability, to connect the PLMN traffic channel (full duplex) to either of these modems.

Looking at the overall Facsimile service architecture in PLMN (figure 5/43.045), half-duplex transmission is initiated in either side of the connection by the actual request on the physical interface (CT109 ON on the local modem), is then continued on the PLMN traffic channel (TCH), and finally terminated on the appropriate modem at the remote side.

On the basis of the above assumptions, considering the Fax Adaptor a finite state machine, the whole Adaptation process can be described by a set of 5 states:

– IDLE state, when the FA is just connected to the GSM-TCH, sending synchronizing patterns over the radio path;

– BCS-REC state, when the FA is receiving data from the V.21 modem (BCS phases);

– BCS-TRA state, when the FA is transmitting data over the V.21 modem (BCS phases);

– MSG-REC state, when the FA is receiving data from the MSG modem (2400/4800/7200/9600/

12000/14400 bit/s);

– MSG-TRA state, when the FA is transmitting data over the MSG modem.

5.2.2 Basic protocol structure

The protocol structure is based on a strictly synchronous approach, using 64 bit fixed length frames; that is each FA actually sends/receives information as 64 bit frames, in sequence and without interruption, during the whole duration of a call; the content of each frame depends on the specific state currently implemented.

The following set of frame types encompasses the full range of capabilities required:

– SYNC frame, explicitly designed to allow synchronization at the remote end, even in the adverse transmission environment like the cellular radio channel; it is a unique frame, used even as idle frame whenever there is no information to be sent over the radio path;

– STATUS frame, intended to carry both state identification codes, along with state specific information; this frame has a unique structured format to allow synchronization checking at remote side; the actual information content is related to the specific state;

– DATA frame which is fully unstructured, and carries Fax coded information during MSG phases.

5.2.2.1 Frame formats

To reduce complexity of finding synchronization over a 64 bit pattern, SYNC frames are obtained by concatenating two 32 bit sync codewords, the second one being the 1’s complement notation of the first one.

The following codes shall be used for sync codewords:

MSB

hex code 3^E375096: first sync codeword

hex code ClC8AF69: second sync codeword

To improve the probability of detecting synchronization, up to 3 errors are tolerate in each single sync codeword; even in this conditions, the false sync probability is quite negligible.

Figure 7/43.045: STATUS frame format

STATUS frames have an internal 8 bit modularity (see figure 7/43.045), where odd octets contain state identification codes (IDENT), and even octets contain status specific information (INFO). IDENT octets are split in two (four bit) fields, each one repeating the same code for status identification:

hex code 1: BCS-REC state

hex code 3: MSG-REC state

hex code 4: MSG-TRA state

To improve the probability of detecting the correct state, up to 3 corrupted codes are tolerate in the total 8 instances repeated within a single STATUS frame. INFO octets contain the same code repeated 4 times within one frame; in particular:

– for BCS-REC state this octet contains 8 bits received from the BCS modem;

– in all other states an idle code (hex code 0F) is inserted.

DATA frames are 64 bit fully unstructured information blocks; the last DATA frame in a Message phase will be truncated to the actual length of the bit stream to be transmitted.

A general rule concerning all the above frames is that most significant bits are transmitted first; in addition, a basic First‑In First‑Out functionality will be implemented in the information passing process.

5.2.3 Protocol description

The above defined Finite State Machine model will be used to fully describe the protocol procedure.

5.2.3.1 IDLE state

In each FA this state implies a continuous transmission of SYNC frames towards the radio path, to allow frame synchronization at the remote end. This state is entered into immediately after the end of the synchronization process over GSM-TCH, and will be returned to whenever loss of synchronization is assumed.

5.2.3.2 BCS‑REC state

The basic function of FA in this state is transferring BCS information from local modem to GSM‑TCH.

Transition to this state is triggered by CT109 ON condition of V.21 modem. Towards the radio path continuous transmission of STATUS frames is performed, according to the format described in the above subclause 5.2.2.1, as soon as 8 bits at least are available from the modem; information received from GSM‑TCH is ignored while CT109 (local modem) is in the ON condition.

Every 8 bits received from the V.21 modem, the appropriate number of STATUS frames (1 or 2 or 4 or 6 depending on the current TCH access rate) will be originated.
Following CT109 (local modem) OFF condition, padding bits (e.g. ls) will be inserted to assemble the last octet.

Monitoring the content of certain BCS frames is required; the DCS frame requires a specific procedure, detailed in the following subclause 5.2.4 in the present document.

5.2.3.3 BCS‑TRA state

The basic function of FA in this state is transferring BCS information from GSM-TCH to its local V.21 modem.

Transition to this state is triggered by reception of BCS-REC code from TCH. Towards the radio path continuous transmission of SYNC frames is performed, according to the format described in the above subclause 5.2.2.1.

The information received from GSM-TCH, after proper decimation (likely a voting algorithm, implementation dependent), is passed to the V.21 modem. Upon detecting again a SYNC frame, the modem is turned OFF and a transition to the IDLE state is performed.

Monitoring the content of certain BCS frames is required; the DCS frame requires a specific procedure, detailed in the following subclause 5.2.4 in the present document.

In addition a supervisoring function is required to check maintenance of synchronization, by examining the inherently structured STATUS frame format. Loss of synchronization will be assumed in case of reception of unstructured STATUS frames, that is, when the repetition mechanism for both the IDENT and the INFO octets does not allow unambiguous result to the voting algorithm. In this case an estimate of the INFO octet value in the received STATUS frames is anyway passed on to the V.21 modem, up to a maximum BCS-TRA duration of 2.5 sec.; at this point, if the correct synchronization has not been recovered yet, the modem is turned OFF and a transition to the IDLE state is performed.

In case of loss of V.110 synchronization on the PLMN side, the FA shall transmit 0s towards the analogue interface, as long as no data is available.

5.2.3.4 MSG‑REC state

The basic function of FA in this state is transferring MSG information from its local modem to GSM-TCH.

Transition to this state is triggered by the MSG modem being trained. Towards the radio path continuous transmission of STATUS frames interleaved with SYNC frames is performed. Actual transfer of Fax coded data over the radio path can be initiated only after the specific acknowledgement is received from the remote side, that is reception of MSG-TRA indication in a STATUS frame, signifying the correct state transition. All data received from the MSG modem will be stored in the FA buffer, to be passed on to GSM-TCH (First-In First-Out mechanism) as soon as this confirmation message is received. Transmission is performed by means of unstructured DATA frames, aligned to the last SYNC or STATUS frame; in this phase, information received from GSM-TCH is ignored.

Following CT109 OFF condition of MSG local modem, after all buffered data are transmitted (the last DATA frame is truncated if necessary), a transition to the IDLE state is performed. In this state the FA waits (meanwhile BCS data received from the local modem are ignored) for a minimum of 5 transmitted SYNC frames, in order to indicate to the remote FA the end of the message.

While waiting for the acknowledgement re‑synchronization shall be performed if necessary, following reception of unrecognisable SYNC frames or unstructured STATUS frames.

In case of 7.2 kbit/s MSG speed, a SYNC frame will be stuffed every 3 DATA frames, to produce the data stream at 9.6 kbit/s. The overall protocol structure will result in multi-frame entities (3 DATA frames followed by a single SYNC frame), continuously sent over the radio path. Similar mechanisms for multislot configurations are specified in table 1a/43.045 for MSG speed of 12 kbit/s.

5.2.3.5 MSG-TRA state

The basic function of FA in this state is transferring MSG information from GSM-TCH to its local MSG modem.

Transition to this state is triggered by reception of MSG-REC code from TCH; towards the radio path continuous transmission of STATUS frames interleaved with SYNC frames is performed.

The MSG modem is trained and a timer (300 m/sec) corresponding to the round trip time over the GSM‑TCH is started. After timeout, loss of synchronization in the information received from TCH, will be assumed as the first Fax coded DATA frame. From the receipt of Message on, continuous transmission of SYNC frames is performed.

All data received from the GSM-TCH will be stored in the FA buffer, to be passed on to the MSG modem (First-In First-Out mechanism) as soon as the modem training terminates (CT106 ON).
From this time on, re-synchronization will be attempted continuously; when an IDLE state is recognized again in the data stream received from the radio path, end of MSG phase will be assumed; then a transition to the IDLE state will be executed, where the FA will wait (ignoring data received from GSM‑TCH) until the buffered information has been fully transmitted to the local MSG modem; the procedure will then proceed in the normal way.

In case of 12.0 or 7.2 kbit/s MSG speed, the above general rule applies as well.

NOTE: In this case, no longer an isolate SYNC frame can be interpreted as the end of MSG phase (transition to the IDLE state of the originating FA at the remote side).

However multi-frame synchronization shall be checked in addition, to remove the SYNC frame stuffed by the originating FA to match the 14.4 kbit/s or 9.6 kbit/s Access Rate over the PLMN. If necessary multi-frame re-synchronization shall be performed.

In case of loss of V.110 synchronization on the PLMN side, the FA shall transmit 0s towards the analogue interface, as long as no data is available.

5.2.4 DCS and TCF processing

Transmission of TCF is performed end-to-end between the two Fax apparatuses, and requires in both FAs a specific routine triggered by DCS command.

As far as the originating FA is concerned, the general procedure as described above for MSG phase (MSG-REC state) applies, but no acknowledgement is required, both at the beginning and at the end, and so no buffering is necessary. That is, just following CT 109 ON condition of the MSG modem, unstructured DATA frames are sent over the radio path, aligned to previous frames; upon CT 109 OFF condition the last frame is sent (truncated if necessary), and a transition to the IDLE state is performed.

As far as the terminating FA is concerned, after passing DCS command and waiting for the appropriate delay (75 m/sec), transition to the MSG‑TRA is executed; here modem training shall be pre-empted and, as soon as the modem is ready for sending (CT 106 ON), loss of synchronization on TCH will be assumed as the first DATA frame containing TCF information and will be passed to the MSG modem. After 1.5 sec. timeout (standard TCF duration), the MSG modem will be turned OFF and the IDLE state entered as usual. Fill information (i.e. logical 0s) will be sent on the local modem if real TCF bit stream is not available.

When DCS frame requires a different Message speed with respect to the actual Access Rate established on the PLMN channel, CMM procedure will be issued (at IWF side only) as detailed in subclause 4.3.2 of the present document, just upon detecting the end of DCS frame, before any other task.

To ensure that the time gap between the DCS and TCF is within 75 +/- 20 ms period as specified in ITU-T/T.30, the training shall be pre-empted in the terminating FA, as defined above.

5.2.5 DCN (disconnect) frame

The FA/MT, upon detection of the DCN frame (see T.30) sent by the local terminal to indicate the end of facsimile transmission, passes this information to GSM-TCH in the normal way and then, initiates the disconnection procedure towards MT, as defined in subclause 6.3/43.045. When the DCN frame is received from GSM-TCH, disconnection procedure is initiated immediately by the FA/MT, as defined in subclause 6.3/43.045.

5.2.6 Clocking

At MS the Fax Adaptor or PLMN facsimile machine will acquire received data bit timing on CT115 (from MT). The transmitter element timing CT114 (from MT) shall be synchronized to CT115.

Since a synchronous terminal adapter function is used, the clock rate over the V.24 interface will always reflect the rate over the radio interface.

In this Teleservice the Network Independent Clocking (NIC) mechanism is not used; to compensate against mismatching between PLMN clock speed and local modem clock speed, a FIFO buffering technique shall be adopted in the FAs on both the PLMN sides. The strategy to manage the buffer queue has a direct impact on the overall delay of the MSG phase, and therefore on T.30 operability in the ensuing post-message phase; basically this procedure is regarded as implementation dependent, and hence is beyond the scope of the present document. A possible implementation is described below, aiming at minimizing the addition delay.

In the originating FA, when the modem speed is lower, according to T.4 (subclause 4.1.3/T.4) a pause may be placed in the message flow by transmitting a FILL sequence (variable string of 0s) between a line of Data and an EOL character. When the modem speed is higher, as no flow control is provided by T.4 coding, the buffer will store excess data resulting from a Fax page transmission.

In the terminating FA the same control means will be exploited.

5.2.7 Timeouts

The overall Fax Adaptation function has no intrinsic timeout, and so relies fully on the timing constraints associated to the end-to-end T.30 procedure.

This means that, no matter of the reference configuration used at the MS, either the "standard" one (figure 2a/43.045) or the "PLMN Facsimile Machine" (figure 2d/43.045), the progress of the Call for this Teleservice will be merely subject to the T.30 typical timing protections, settled externally (physically and/or functionally) with respect to the procedure as described above.

5.3 Specific TAFs for facsimile service (T.30/A)

The optional error correction procedure, as defined in ITU-T T.4 and T.30 (annex A) Recommendations may be fully supported, provided some specific features are added to the Fax Adaptation procedure as resulting from the previous subclause 5.2/43.045.

These features are relevant to:

– additional HDLC frame to be detected;

– handling of the Message phase;

– modification of the Channel rate (CMM request);

The Error Correction mode is entered upon detection of the relevant bits in the DIS/DTC frame.

If the FA does not support the ECM, the relevant bit in the DIS has to be set to zero by the FA.

5.3.1 Frame detection

During the BCS phases, beside the frames enlisted in subclause 4.2.1.1/43.045, an additional frame (CTC) has to be detected, as it fixes the retransmission strategy chosen by the Fax transmitting terminal.

5.3.2 Message phase

The same buffering approach as for standard T.30 procedure (see subclause 5.2.3.1/43.045) will be exploited to overcome the mismatching in clock speed between PLMN and the local modem; in this case however the HDLC flag code (hex 7E) will be used to control the buffer level, as the Fax coded data are structured in HDLC frames.

5.3.3 Additional CMM request

While in BCS the CTC frame has to be detected, due to the possible (not mandatory) request of changing the transmission speed to a fallback bit-rate.

The CMM request towards the PLMN (when appropriate) is issued at the IWF side, by exploiting the bit-rate indication in the CTC frame, similar to the indication in DCS frame.

Upon detecting the CTC frame, either sent by the PSTN or by the MS, provided the transmission speed indication is different from the existing Channel rate, as soon as the end of frame is detected, CMM request is issued, using exactly the same procedure as described for DCS in subclause 4.3.2 of the present document

If a 7,2 kbit/s MSG speed is requested in the CTC frame, no CMM request is necessary, and the specific procedure described in subclause 4.2.3.4 applies, using the same 9,6 kbit/s Access Rate over the PLMN channel.